diff --git a/lib/Kconfig b/lib/Kconfig
index 5115d04..1048068 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -61,6 +61,9 @@
 	  size-constrained envrionments even this may be too big. Enable this
 	  option to reduce code size slightly at the cost of some speed.
 
+config RBTREE
+	bool
+
 source lib/dhry/Kconfig
 
 source lib/rsa/Kconfig